Different catalysts comprising monovalent magnesium and a transition metal are already known and they have been used before for the polymerization of olefins. U.S. Pat. No. 3,048,571, for example, describes catalysts which are combinations of monovalent magnesium compounds, which may be obtained either by electrolytic reduction or by pyrolysis of an aryl-magnesium halide, and polyvalent compounds of metals of sub-groups IV to VI of the Periodic Table, the molar ratio of the monovalent magnesium to said polyvalent compounds ranging between 0.1 and 10. These catalysts are used for the polymerization of olefins in inert liquid organic solvents, at temperatures ranging between 0.degree. C. and 350.degree. C. and under pressures of 1 to 300 bars. British Pat. No. 1,150,640 also describes catalyst mixtures comprising a transition metal compound of groups IV to VIII of the Periodic Table and a monovalent compound of magnesium obtained by thermal decomposition under reduced pressure and at temperatures of 190.degree. C. to 240.degree. C. of an aryl-magnesium halide prepared in the presence of an ether or a hydrocarbon, the molar ratio of the monovalent magnesium compound of said transition metal compound ranging between 10 and 50.
In the above patents the use of these catalysts is said to have numerous advantages because it evades the danger of employing inflammable compounds which sometimes ignite spontaneously such as trialkylaluminum (See column 2, lines 17-21 of U.S. Pat. No. 3,048,571). However, the catalytic yields in the polymerization are weak and it is necessary to wash the polymer in order to obtain commercial qualities.
Catalysts obtained by contacting a titanium or vanadium halide with anhydrous magnesium dihalide are also known. They are described, for example, in French Pat. No. 2,023,789. The use of these catalysts for the polymerization of ethylene under high pressures and temperatures allows fairly high yields while avoiding polymer washing. The use of these catalysts, however, gives ethylene polymers having a proportion of very high molecular weights which is not sufficient, particularly in the case of polymers having a melt index lower than 2 dg/mn that are to be shaped by extrusion-blowing. Those polymers which have a sufficient proportion of very high molecular weights also have weak mechanical properties, particularly their breaking strength.